Replacement of corroded equipment can be a major expense in an industrial process, both from the standpoint of equipment cost and from the standpoint of lost production during the replacement process, as well as costs for removal and disposal of the corroded equipment. In addition, maintenance costs for equipment in a corrosive environment may be high.
A number of approaches are utilized to reduce the effects of corrosive substances on metal equipment. These include fabricating the equipment from corrosion resistant materials such as titanium, zirconium or tantalum; coating or lining the equipment with corrosion resistant materials such as glass; and adding corrosion inhibiting substances to the corrosive materials. Use of corrosion resistant metals and coating of equipment with inert materials can be expensive.
When corrosion inhibiting substances are employed care must be taken to fully evaluate any proposed metal/corrosion inhibitor system, that is, the metal, the corrosive material, the inhibitor, and other components which may be present, in order to avoid unexpected results, the most important being failure to inhibit corrosion. For example, fluoride ions accelerate the dissolution of titanium oxide. Therefore, whenever fluoride ions are present, oxidizing agents generally do not work well as titanium corrosion inhibitors. In some cases low concentrations of corrosion inhibitors actually increase the corrosion rate. They only function as inhibitors at concentrations above what is known as the critical value.
In highly corrosive environments, such as occur in the presence of sulfuric/hydrocyanic acid mixtures, corrosion resistant metals are often used. Unfortunately, such acid mixtures are sufficiently corrosive that even when corrosion resistant metals are used unacceptable corrosion often occurs, especially at elevated temperatures which occur, for example, in distillation columns during distillation. For that reason, corrosion inhibitors are typically added to such mixtures.
Corrosion resistance of many of the common metals, including aluminum, iron and steel, titanium, and zirconium, is through formation of a metal oxide layer on the metal's surface. In environments where water or oxygen are present, such metals regenerate metal oxide layers spontaneously. In more aggressive environments, such as in the presence of acidic mixtures, the metal oxide layer may be depleted faster than the metal can oxidize to spontaneously regenerate it. In those cases, oxidizing agents are good choices for corrosion inhibitors because they increase the rate of oxide layer regeneration.
The most commonly used oxidizing agent inhibitors are copper salts such as copper sulfate. These salts have the advantages of having good activity as corrosion inhibitors, ready availability, solubility in aqueous solutions, and reasonable cost. Unfortunately, they also have a significant drawback. They are considered environmentally detrimental and, therefore, are difficult to dispose of in an environmentally acceptable manner. Thus, there is a need for environmentally acceptable alternatives to copper salts as corrosion inhibitors in metal vessels exposed to acidic mixtures.
I. P. Anoshchenko et al., in Werkstoffe und Korrosion 25. Jahrg. Heft 10/1974 reports that in addition to copper salts, iron salts are known to inhibit corrosion of titanium by acidic solutions such as sulfuric, hydrochloric, and phosphoric acids. However, we expected that iron salts would be ineffective for inhibiting metal corrosion in the presence of sulfuric acid/hydrocyanic acid mixtures due to the formation of Prussian blue or other iron cyano complexes. Such complexes are produced by the precipitation of ferrous ferrocyanide from a soluble ferrocyanide and ferrous sulfate at acidic pH. Iron cyano complexes are known to be insoluble in water and, therefore, would be expected to be unavailable to act as oxidizing agents on the metal surfaces in aqueous environments.